In U.S. Pat. No. 4,639,884, a unique solution to a longstanding problem of measuring precisely the velocity and position of a servo system over a wide range of speeds is disclosed. In accordance with the principles of the invention described in this patent, which is incorporated herein by reference, a servo controller applied to the output shaft of a servo motor employs a rotary-position sensor to produce a two-bit digital signal in Gray Code. As the shaft rotates through a fixed angle, the quadrature signal changes to produce a "quadrature transition", a term used to denote the movement of the rotary-position sensor through an angle that is sufficient to cause the sensor's quadrature output to advance to the subsequent Gray Code pattern. Thus, movement of the shaft (axis) produces a pair of signals in quadrature with transition, expressible in Gray Code for each cycle as 00, 01, 11, 10 and then back to 00. It also produces, as from a clock, a large number of pulses per group of quadrature transitions at all speeds of the shaft. The method continues by reading the number of clock pulses between two selected quadrature transitions, and then determines the velocity by dividing the total number of quadrature transitions between the selected transitions by the total number of clock pulses during the same time.
In the ideal servo system, the overlapping square wave transitions of the feedback system fall precisely at rotary or linear distance intervals of the servo axis. While, for the most part, the approximation is sufficiently precise, there are situations in which a low precision level limits servo axis performance even when using the method and apparatus of U.S. Pat. No. 4,639,884. One intent of the present invention is to improve the measurement method and apparatus to better measure the non-ideal feedback signals, particularly for servo systems subjected to an environment having electrical noise. The term "electrical noise" refers to unwanted transitions of the overlapping square wave signals of the feedback loop. In servo systems, electrical currents in either the feedback loop or the actuator system can inductively or capacitatively couple to circuits within the feedback loop and generate unwanted transitions. The result of the noise is that the transitions of the feedback system do not precisely correspond to the actual motion of the axis.
Significant electrical noise levels are most common in systems with electric actuators such as electric motors. In such cases the power to move the axis is derived from electricity. Pulse-width-modulated semiconductor switches are commonly used to regulate the amount of power delivered to the electric motor. The switching of inductance of the motor with the semiconductors generates large electric currents and magnetic fields. It is these currents and fields which are most often the source of electrical noise in the servo system.
Other sources of electrical noise in servo systems include currents and fields generated by equipment and machines surrounding the servo system. One source common to most servo systems is noise in the feedback system itself. In order to reduce the effects of internal noise, most feedback systems include some amount of electrical hysteresis which manifests itself as a tendency of the overlapping square waves to settle in a stable manner. Electrical hysteresis in the feedback system adjusts the threshold of the sensing element based upon the direction that the threshold is being approached. Some non-ideal feedback systems have low or even negative electrical hysteresis which increases the system's tendency to oscillate about two adjacent Gray Code patterns of the overlapping square wave quadrature signals.
An ideal feedback system should produce transitions of its overlapping square wave signals if and only if the servo axis moves, and also should produce no transitions of its overlapping square wave signals if the axis does not move. In particular, the ideal feedback system should unambiguously signal the change from one discernible position to the next at very low velocities by causing a single transition of the overlapping square waves per position interval or count. In a manner analogous to a wheel of fortune, which is bistable between one number and the next, the ideal feedback system should slow and stop at a given position without continuing to bounce between one of two Gray Code quadrature states or position counts.